Re: [PATCH 3/4] virtio_ring: introduce a per virtqueue waitqueue

[Jason Wang <jasowang@redhat.com>]

On Wed, Dec 28, 2022 at 2:34 PM Jason Wang <jasowang@redhat.com> wrote:
>
>
> 在 2022/12/27 17:38, Michael S. Tsirkin 写道:
> > On Tue, Dec 27, 2022 at 05:12:58PM +0800, Jason Wang wrote:
> >> 在 2022/12/27 15:33, Michael S. Tsirkin 写道:
> >>> On Tue, Dec 27, 2022 at 12:30:35PM +0800, Jason Wang wrote:
> >>>>> But device is still going and will later use the buffers.
> >>>>>
> >>>>> Same for timeout really.
> >>>> Avoiding infinite wait/poll is one of the goals, another is to sleep.
> >>>> If we think the timeout is hard, we can start from the wait.
> >>>>
> >>>> Thanks
> >>> If the goal is to avoid disrupting traffic while CVQ is in use,
> >>> that sounds more reasonable. E.g. someone is turning on promisc,
> >>> a spike in CPU usage might be unwelcome.
> >>
> >> Yes, this would be more obvious is UP is used.
> >>
> >>
> >>> things we should be careful to address then:
> >>> 1- debugging. Currently it's easy to see a warning if CPU is stuck
> >>>      in a loop for a while, and we also get a backtrace.
> >>>      E.g. with this - how do we know who has the RTNL?
> >>>      We need to integrate with kernel/watchdog.c for good results
> >>>      and to make sure policy is consistent.
> >>
> >> That's fine, will consider this.

So after some investigation, it seems the watchdog.c doesn't help. The
only export helper is touch_softlockup_watchdog() which tries to avoid
triggering the lockups warning for the known slow path.

And before the patch, we end up with a real infinite loop which could
be caught by RCU stall detector which is not the case of the sleep.
What we can do is probably do a periodic netdev_err().

Thanks

> >>
> >>
> >>> 2- overhead. In a very common scenario when device is in hypervisor,
> >>>      programming timers etc has a very high overhead, at bootup
> >>>      lots of CVQ commands are run and slowing boot down is not nice.
> >>>      let's poll for a bit before waiting?
> >>
> >> Then we go back to the question of choosing a good timeout for poll. And
> >> poll seems problematic in the case of UP, scheduler might not have the
> >> chance to run.
> > Poll just a bit :) Seriously I don't know, but at least check once
> > after kick.
>
>
> I think it is what the current code did where the condition will be
> check before trying to sleep in the wait_event().
>
>
> >
> >>> 3- suprise removal. need to wake up thread in some way. what about
> >>>      other cases of device breakage - is there a chance this
> >>>      introduces new bugs around that? at least enumerate them please.
> >>
> >> The current code did:
> >>
> >> 1) check for vq->broken
> >> 2) wakeup during BAD_RING()
> >>
> >> So we won't end up with a never woke up process which should be fine.
> >>
> >> Thanks
> >
> > BTW BAD_RING on removal will trigger dev_err. Not sure that is a good
> > idea - can cause crashes if kernel panics on error.
>
>
> Yes, it's better to use __virtqueue_break() instead.
>
> But consider we will start from a wait first, I will limit the changes
> in virtio-net without bothering virtio core.
>
> Thanks
>
>
> >
> >>>